We have established assays to examine expression of the drug resistance phenotype using flow cytometric analysis of intact single cells. There are significant advantages to this approach, including the ability to derive rapid, semiquantitative data on tens of thousands of cells and the ability to study the heterogeneity of drug resistance within clinical specimens. The techniques developed include analysis of P-glycoprotein expression using the monoclonal antibody MRK-16, flow cytometric drug cytotoxicity assays based on the esterase activity of surviving cells, expression of thymidylate synthase using monoclonal antibody developed by Drs. Johnston and Allegra, and folate receptor studies using fluoresceinated methotrezate or anti-folate receptor antibody with Drs. Chung and Elwood. In the past year, we have used these techniques in a wide variety of applications in collaborative studies. These applications include examination of the regulation of P-glycoprotein expression in breast cancer cell lines transfected with the gene for protein kinase C alpha or protein kinase C gamma with Drs. Ahmad and Glazer; quantitation of intracellular thymidylate synthase levels in parental and 5-FU- resistant colon and breast cancer cells; examination of the expression of the folate receptor in cells transfected with a folate receptor expression vector with Drs. Chung and Elwood; and analysis of the role of myeloperoxidase in vinca resistance in acute myeloblastic leukemia with Drs. Schlaifer and Myers.